1. Field of the Invention
The invention relates to a method of manufacturing a laminated ceramic capacitor, and more particularly to a method of manufacturing such a capacitor using an electrically conductive paste used for the formation of an internal electrode of a laminated ceramic capacitor which uses lead system composite perovskite oxide as a dielectric substance.
2. Description of the Related Art
The laminated ceramic capacitor as aforementioned is manufactured generally in the following steps. First, lead system composite perovskite oxide powder as a ceramic dielectric substance powder is mixed with an organic binder and a solvent, and then the mixture is formed into a sheet by means of a casting process. Then, on the green sheet are formed electrically conductive paste layers which are to work as internal electrodes, by means of a screen printing process. After a certain number of the layers have been deposited, they are compressed to bond with each other. Then, the thus laminated layers is cut into individual capacitors. Then, the individual capacitors are fired so that a ceramic dielectric substance and metal powder in the paste are simultaneously sintered. Finally, an external electrode is formed.
In the above-described process, a step of simultaneously firing the dielectric ceramic in the form of a sheet and the metal powder in the electrically conductive paste to be sintered is indispensable to a method of manufacturing a laminated ceramic capacitor.
A laminated ceramic capacitor can have desired properties by forming ceramic dielectrics and internal electrodes closely integral with each other by means of the above mentioned firing step. Accordingly, it is quite important, in the firing step to avoid so-called "delamination of layers", which means a phenomenon wherein a ceramic dielectric is peeled off from an internal electrode in a laminated structure after the firing step, in order to obtain a capacitor having stable and reliable. The delamination of layers occurs when a stress acting on an interface of the metal (internal electrode) and the ceramic (dielectric substance) is greater than a bonding strength between them. In view of this point, the following are considered to be problems frequency found in a firing step.
1. A ceramic can have only a weak bonding strength with a metal, and hence it is impossible to strongly bond a ceramic to a metal.
2. Since there is a difference in a volume shrinkage due to firing between a ceramic and a metal, a stress would be generated at an interface of them particularly when the temperature is being raised in a firing step.
3. Because of a great difference in a thermal expansion coefficient between a ceramic and a metal, a volume expansion and shrinkage due to an increase or decrease in temperature during firing would entail a large stress.
Thus, in order to avoid delamination of layers between internal electrodes and dielectric substances in a laminated ceramic capacitor, an electrically conductive paste used for an internal electrode is required to have a great bonding strength with a ceramic dielectric and to generate a small stress.
A conventional electrically conductive paste used for a laminated ceramic capacitor is a mixture of metal powder including at least silver powder such as a mixture of silver powder and palladium powder, and a vehicle comprising an organic binder and a solvent. The mixture is well mixed by means of three rolls, and then used as the paste.
As aforementioned, in a conventional method of manufacturing a laminated ceramic capacitor, an electrically conductive paste including the mixture of metal powder including silver powder and a vehicle comprising an organic binder and a solvent has been used for formation of internal electrodes. However, an electrically conductive paste is required to have a characteristic by which a greater bonding strength would be generated between an internal electrode and a ceramic dielectric substance, and also by which a smaller stress would occur at an interface between an internal electrode and a ceramic dielectric substance, in order to enhance the stability and reliability of revelation of the capacitor characteristic.
In view of such a demand, there has not been suggested an electrically conductive paste manufactured only for a laminated ceramic capacitor. However, it is possible to solve the problems of the weakness of a bonding strength and the largeness of a stress by using an electrically conductive paste modified for another objects.
For instance, Japanese Unexamined Patent Public Disclosure No. 63-115691 discloses an electrically conductive paste having an improved bonding strength with a non-oxide ceramic such as AIN. In this Disclosure, an organic titanium family adhesion promotion agent or a titanate coupling agent is added to a conventional active silver solder paste, and hydrolysis of the titanate coupling agent with water absorbed in a surface of a ceramic substrate is used to improve the bonding strength. Though the electrically conductive paste aims to strengthen a bonding strength between high thermally conductive ceramic veneers or between such a ceramic veneer and a metal, the electrically conductive paste can be used as an internal electrode to thereby obtain a greater bonding strength between an internal electrode and a ceramic dielectric substance than that obtained by a lo conventional electrically conductive paste.
Japanese Unexamined Patent Public Disclosure No. 63-283184 has suggested an electrically conductive paste including conductive compound containing silver powder and palladium powder as indispensable constituents, and rhodium powder and/or organic rhodium compound. These ingredients are distributed in vehicle. The object of the Disclosure is to provide a paste which is to be used for forming electrically conductive patterns on a fired veneer glass substrate or a fired glass ceramic substrate, and which shows a good wettability to solder and does not degrade adhesion between a conductive compound and a substrate. As later explained in detail, the paste suggested in the Disclosure contains rhodium, and hence, when the paste is used for forming internal electrodes of a laminated ceramic capacitor, a volume shrinkage thereof due to firing in a firing step is similar to that of a ceramic dielectric. As a result, it is expected to obtain an advantageous effect that it is possible to avoid an occurrence of a stress produced due to a difference in a volume shrinkage generated due to firing between internal electrodes and ceramic dielectrics.
However, even if either paste disclosed in the above mentioned two Disclosures is used, only one of the two factors, namely a bonding strength and a stress which cause a delamination of layers in a step of firing a laminated ceramic capacitor, can be improved, and hence it is impossible to sufficiently avoid a delamination of layers.
For instance, the electrically conductive paste disclosed in the first mentioned Disclosure can strengthen a bonding strength between ceramic dielectrics and internal electrodes, however, a stress may be larger than a bonding strength due to a difference in a volume shrinkage due to firing or a difference in a thermal expansion coefficient.
On the other hand, the electrically conductive paste disclosed in the second Disclosure can reduce a possibility of production of a stress due to a difference in a volume shrinkage due to firing. However, a bonding strength remains as it is, and hence if a stress produced due to a difference in a thermal expansion coefficient is larger than a bonding strength, there would occur a delamination of layers while a temperature is being lowered in a firing step, a local concentration of stress due to a dispersion in a bonding strength between an internal electrode and a ceramic dielectric, and a microcrack due to the local concentration of stress.
In addition, the electrically conductive paste disclosed in the first mentioned Disclosure has to be fired in a vacuum or an inert gas atmosphere. On the other hand, if lead system composite perovskite oxide used for a laminated ceramic capacitor to which the invention is directed is fired in a vacuum or in an inert gas atmosphere, the dielectric characteristic thereof is influenced by anoxia, and thereby a dielectric constant thereof is lowered. In conclusion, the electrically conductive paste disclosed in the first mentioned Disclosure is not practically applicable to the manufacturing of a laminated ceramic capacitor in which an electrically conductive paste and a ceramic dielectric have to be simultaneously fired.